This disclosure relates to investment casting. Investment casting is known and used to cast metallic components with relatively complex geometries. For example, gas turbine engine components, such as airfoils, are fabricated by investment casting. For cast components that have internal passages, the internal passages can be formed using a core that represents a positive projection of negative features that are to be formed in the casting process. A wax pattern is provided around the core in the geometry of the component to be cast. A refractory shell is formed around the wax pattern and the wax is then removed to form a mold cavity between the core and the shell. Molten metal is poured into the cavity. After solidification of the metal, the shell and core are removed using known techniques, to release the cast component.